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Forties Field bigger than perceived

December 2005

APACHE'S NORTH SEA team in Aberdeen is preparing the legendary Forties Field for the next 20 years of production.

Apache抯 subsurface team has concluded that the largest field ever discovered in the United Kingdom sector of the North Sea contained more original oil in place than the previous operator proposed � as much as 800 million barrels more, in fact. That means more oil still to be tapped, beyond the 2003 estimates that formed the basis for Apache抯 acquisition.

Less than three years after Apache arrived on the scene, Forties facilities upgrades include: new generators with enough electric generating capacity (56 megawatt) to power a small city; a new 搑ing main� to distribute gas and power throughout the field; new cranes; modernized drilling rigs on four platforms; and new electric oil export pumps.

Additionally, major projects are under way for produced water re-injection, additional high-pressure gas-lift systems, new centralized and updated control equipment on each platform, conversion of the Bravo platform to normally unmanned status, and upgraded accommodation facilities on the other three main platforms.

The result of these facilities improvements are many: per-barrel operating costs are half what they were in 2003; emissions will be reduced by 46 percent upon completion of the ring main and electric generation projects; and field efficiency has increased to 88 percent, up from 70 percent.

With the improved efficiency and additional drilling, Apache has nearly doubled production, reaching as high as 81,000 barrels per day, and added approximately 80 million barrels of proved reserves.

The 搕opside� improvements help, but the real future of any oil field is thousands of feet below the surface. While BP had identified 19 prospect locations of adequate size to drill, Apache has drilled more than 30 wells since 2003, and the inventory contains about 20 more locations at ready-to-drill status. In total, since Apache抯 acquisition of the field, well over 100 potential drilling targets have been identified. 

In a large field like Forties, which has produced 2.5 billion barrels of oil, understanding the geology and picking locations to drill for production or water injection is a complex task that involves correlating information from many sources � well logs, seismic data, the performance of wells drilled in the field over the years, and individual formation zones � in a way that offers a complete picture. In the summer of 2005, additional seismic was acquired to add to the fourth dimension � or time-elapse � reservoir understanding that has worked well to date with drilling target identification.

In the case of the Forties Field, the equivalent of five professionals working full-time for a year and a breakthrough software integration package from Schlumberger � Petrel � helped provide a clear picture. The new, full-field reservoir model was built to help Apache identify non-drained areas of the field and plan for improved water injection, which will help maintain the pressure in the field and keep the oil flowing.

Apache抯 geoscientists now believe that the field was much larger than thought when it was discovered. BP抯 experts estimated the field had about 4.2 billion barrels of 搊riginal oil in place.� Based on the new analysis, that number has grown to about 5 billion barrels, said John Crum, executive vice president and managing director of Apache North Sea. 

揑n a traditional reservoir study, the geologists, geophysicists, production engineers and reservoir engineers work on different aspects of a well抯 geology and performance and then try to integrate what they know,� said Alan Clare, manager of exploration technology, who coordinated Apache抯 work to build a Forties Field model.

Apache decided to use Petrel because it permitted all of the data to be integrated into the unified model quickly and efficiently and enables the Aberdeen technical team to update the model to reflect current production and well results.

揂lthough we take the geologic model built in Petrel to other software packages to conduct fluid flow simulation, the team can still use Petrel to visualize the results and make decisions,� explained Sheldon Plahn, Apache North Sea Engineering manager. 

揟he Forties Field covers 36 square miles � an area the size of Manhattan � and the oil-bearing formation is as much as 600 feet thick,� said Phil Rose, senior staff geologist in Aberdeen. 揑t has four stacked channel sand complexes with detailed variations that must be understood to verify non-drained areas or targets. The fact that we have a working model for such a large field is a breakthrough technology.�

Schlumberger has worked closely with Apache to expand the Petrel integrated package application beyond Forties to other projects and regions because the company knows that Petrel will be tested in 搑eal world� conditions due to Apache抯 typical drilling pace, not just in a research setting, Clare said.

The new Forties Field model has already demonstrated its value according to David Allard, Apache North Sea Exploration manager: 揥e drilled Target 51, in the Delta-Echo channel, but it had been swept [drained]. We planned to sidetrack the well to a contingency target in the Bravo channel, but the new field model indicated an unswept area a quarter of a mile away. We found 20 meters (65 feet) of pay, and the well came on at 3,000 barrels per day.�/P>

With the refurbished platform drilling rig improvements and rotary steerable technology, Apache is reducing well costs and now can reach previously unavailable, non-drained targets farther from the platforms. The reduced well costs allow Apache to economically drill smaller targets as well, Allard added.

揥e抳e developed a good field model of the remaining oil in place,� Crum said. 揥hat does this mean in terms of eventual ultimate recovery? Only time will tell � we抳e got to drill more wells. However, I抦 confident that the combination of improved reservoir understanding, lower costs and high oil prices will yield significant reserve increases over the next several years.�/P>

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Published periodically for employees of Apache Corporation by the Public & International Affairs Department.
Editor: Lisa Nutting
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